The present invention relates to fibers of polypropylene, and copolymers of propylene with a minor amount of a copolymerizable alpha-olefin monomer X (P-co-X), for example ethylene (P-co-E), stabilized against degradation by gamma-radiation. Fibers of polypropylene and such copolymers, individually and collectively referred to herein as "propylene polymer fibers" (PP fibers), upon exposure to gamma-radiation without being stabilized, develop objectionable color and lose their desirable physical properties because of degradation. More particularly, the invention relates to stabilized PP fibers, and to woven and non-woven articles made from PP fibers, before and after they are exposed to a preselected level of gamma-radiation.
Recent advances in PP technology have been instrumental in propagating the use of the polymer for shaped articles for a wide variety of uses. The chemical inertness and lack of toxicity of PP fibers, their low weight, and the relatively low cost of producing such articles of arbitrary size and shape, makes the PP fibers peculiarly well-adapted for use in the medical and institutional maintenance fields. Thus, PP is the material of choice for laboratory filtration fabric, and a variety of disposable articles including curtains, bedsheets, surgical gowns and the like. Such articles, stabilized with hindered phenols, were routinely sterilized with ethylene oxide but its toxicity resulted in its gradual withdrawal as a sterilant. Ethylene oxide has been replaced by exposure to gamma radiation having an energy level in the range from about 0.5 to about 5 Mrad (megarads) for a short period of time, from about 1 min (minute) to about 12 hr (hours), the higher the intensity of radiation, the shorter the time required.
The problem is that PP fibers degrade rapidly when so sterilized, developing an objectionable yellow color, and suffering a severe loss of physical integrity, the higher the intensity of radiation, the worse the degradation. The degradation, referred to as "oxidative degradation", is particularly noticeable upon storage of a sterilized article at room temperature, the longer the storage period, the worse the degradation.
Considerable effort has been devoted, in the recent past, to the study of the details of the mechanisms and kinetics of the oxidation, with the expectation that such understanding will lead to the correct choice of an effective stabilizing additive (stabilizer) which will control the chemistry responsible for degradation. In an article titled "Recent Developments in the Oxidative Degradation of Polypropylene by Gamma Radiation" by Wiles, D. M. and Carlsson, D. J., theoretical equations are presented to explain the mechanisms and kinetics. They concluded that gamma radiation of PP results in thermal oxidation which is qualitatively very similar to oxidation induced by other means, e.g. to photo-oxidation. The same reactive species are involved and although they will be distributed rather differently, the same oxidation products are formed. Affirmation of this conclusion is provided by the improved gamma-radiation stability of PP stabilized with conventional hindered phenols, but at the expense of highly visible and undesirable discoloration.
The difference in the frequency of the wavelengths of uv light and gamma radiation, coupled with the great difference in their energy levels, makes their relative effects upon a polymer unpredictable. The differences go far deeper than the expected differences relating to their relative penetration into the polymer, as evidenced by the fact that some structurally similar compounds are far less effective stabilizers than others which appear to have all the necessary perquisites of the more effective stabilizers.
Notably, the effects of gamma radiation on PP are readily distinguishable over the effects of such radiation on other polymers (polystyrene "PS", say). PS has been found to be more stable (see "Stabilizers in gamma-irradiated polypropylene" by Horng, P. and Klemchuk, P., Plastics Engineering April '84, pp 35-37), as are several other polymers. They concluded that the high susceptibility to gamma-degradation decreed that the fate and effectiveness of stabilizers can only be assessed by determining whether they are being consumed in the irradiation process or being simply decomposed by gamma radiation. Further, they developed an experimental technique to evaluate the stability of four major genera of additives, namely an antioxidant (AO) such as tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane, a hindered amine (HA) such as 2,2,6,6-tetramethyl-4-piperidyl sebacate, a phosphite such as tris(2,4-di-tert-butyl phenyl)phosphite, and a thioester such as distearyl thiodipropionate. Their study indicated that the mechanical and molecular weight data confirmed that the residual AO or HA in the irradiated PP maintained the physical integrity of the samples by reacting with the propagating radicals and/or by decomposing peroxides. The phosphite and thioester additives were less effective because they behaved as secondary stabilizers and were completely consumed during radiation. But, they offered no basis for choosing an effective HA or AO, other than by repetitive trial and error.
Tests with various HAs for color development in gamma-irradiated PP showed that HA alone did not cause significant discoloration and was the leading choice for medical end-products which were color or appearance-sensitive. Thus, HA was the best overall stabilizer for PP gamma-sterilizable products. Again, however, there is nothing to indicate how to choose the more effective among HAs, other than trial and error.
Yet, British Patent GB 2,043,079 discloses that PP was stabilized against gamma-radiation by incorporating a phenolic AO containing an isocyanurate group, preferably in conjunction with a thiosynergist and a stearate, clearly contradicting the foregoing conclusions. However, there is no indication as to the extent of discoloration suffered.
Of particular interest is the peculiar gamma-stabilization effect of N-(substituted).alpha.-(3,5-dialkyl-4-hydroxyphenyl)-.alpha.,.alpha.-disub stituted acetamides in which one of the substituents on the N atom is a 2-piperazinone group. More correctly, the compounds are "N-(substituted)-1-(piperazin-2-one alkyl)-.alpha.-(3,5-dialkyl-4-hydroxyphenyl)-.alpha.,.alpha.-substituted acetamides", which are hereinafter referred to as "3,5-DHPZNA" for brevity. This 3,5-DHPZNA stabilizer is disclosed in U.S. Pat. No. 4,780,495 to John T. Lai, for its uv-light stabilization in PP, and, because of the presence of a polysubstituted piperazinone (PSP) group in the molecule, was tested in PP plaques for such gamma-ray stabilization effectiveness as it might have. The plaques deteriorated rapidly.
Since the majority of PP articles are extruded or molded shapes other than fibers, most testing for gammaray stabilization is conventionally done with plaques, not fibers, because plaques are more conveniently prepared. Generally, if a stabilizer is effective in fibers it is effective in plaques, but the opposite is not true. It was only by chance that the gamma-ray stabilization of 3,5-DHPZNA was also tested in PP fibers, or its effectiveness in this particular application would have gone unnoticed. It is not known why PP fibers are stabilized with 3,5-DHPZNA against gammas far more effectively than are PP plaques, but PP fibers are. Moreover, 3,5-DHPZNA-stabilized PP can be extruded from a spinneret without the aid of another stabilizer for antioxidation protection. Which is why 3,5-DHPZNA is the sole stabilizer of choice for woven and non-woven goods made with PP fibers.
U.S. Pat. No. 4,797,438 to Kletecka et al discloses that hindered amines with a specific structure known to exhibit excellent uv stabilization in numerous host polymers without notably distinguishing one polymer from another as far as their relative susceptibility to uv stabilization is concerned, are surprisingly effective to stabilize PP against degradation by gamma-radiation. Moreover, such stabilization extends to articles of arbitrary shape, including fibers, and these amines are more effective when used without an AO, phosphite or thioester. It was known, however, that PP fibers stabilized with commercially available hindered phenol AOs such as Goodrite.RTM. 3114 effectively withstood sterilization with gammas, albeit with high discoloration. It appeared that the presence of an AO did little more than contribute to the undesirably high level of discoloration known to be associated with AO-stabilized PP which is sterilized with gamma radiation.
The peculiarly distinguishing structural feature of the stabilizers in the Kletecka et al composition, is that they, like 3,5-DHPZNA, contain as an essential portion of their basic structure, a PSP having an N.sup.1 -adjacent carbonyl in the PSP group, and at least the C.sup.3 (carbon atom in the 3-position in the ring) has two substituents (hence "polysubstituted or substituted"), which may be cyclizable, that is, form a cyclic substituent. But unlike 3,5-DHPZNA, those stabilizers do not contain a hindered phenol group in the same molecule.
Though 3,5-DHPZNA compounds referred to in the aforementioned '495 Lai patent were known to be excellent UV stabilizer in colorless organic materials when used in combination with antioxidants, there was nothing to suggest that its incorporation in PP, alone among other polymers tested, in the absence of any conventional hindered phenol antioxidant, and preferably also in the absence of a phosphite or thioester stabilizer, would provide effective stabilization against gamma-radiation.